Oscillation circuits that use solid oscillators such as crystal oscillators are widely used in electronic equipment, and are used as a generating source for the reference-clock signal of the semiconductor device. Of this kind of oscillation circuit, circuits as shown in FIG. 6 having a construction in which a crystal oscillator XL and resistor R101 are connected in parallel between the input and output of an inverter circuit in which an N-type MOS transistor MN101 and P-type MOS transistor MP101 are connected in series, a capacitor C101 is connected to the input terminal, and a capacitor C102 is connected to the output terminal, are widely used. Also, several circuits are known in which depending on the purpose various changes are made to this basic circuit.
For example, in patent document 1, a semiconductor device is disclosed in which the required layout surface area of the oscillation circuit built in a microcomputer is reduced, and the operation thereof is stabilized. It is constructed so that a protection resistance located between the output terminal of a first CMOS logic circuit and a corresponding external terminal is done away with, and the drive performance of the first CMOS logic circuit is reduced. Also, together with being able to shorten the wiring between the output terminal of the first CMOS logic circuit and the corresponding external terminal, reduce the wiring resistance of that wiring, and further reduce the drive performance of the first CMOS logic circuit, it is possible to shorten the overall wiring length of the oscillation circuit as well as suppress the coupling noise between the wiring and the noise of the power supply. As a result, it is possible to reduce the required layout surface area and stabilize the operation thereof.
Also, in patent document 2, an oscillation circuit is disclosed in which power consumption and noise are reduced by optimizing the drive performance of the inverting amplifier when a crystal oscillator is connected. This oscillation circuit includes a drive-performance-selection circuit that detects an optimum drive-performance level of the inverting amplifier according to the element characteristics of the crystal oscillator, and sets the drive performance of the inverting amplifier based on the detection result. In other words, the circuit comprises a downcounter or upcounter that sets the drive-performance level of the inverting amplifier when a crystal oscillator is connected; it counts down or counts up to find the starting oscillation level, and sets the drive performance.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-A-5-343919
[Patent Document 2]
Japanese Patent Kokai Publication No. JP-P2000-332536A